Senate Judiciary Chairman Chuck Grassley sent a letter to a political opposition research firm Monday, requesting information about who funded an unsubstantiated dossier on President Donald Trump during the 2016 presidential campaign.

Grassley asked the firm, Fusion GPS, for more details on British spy Christopher Steele, who was hired to investigate Trump at the time.

Additionally, he requested information about who initially hired Fusion GPS and whether the firm was in touch with the FBI or Justice Department.

“When political opposition research becomes the basis for law enforcement or intelligence efforts, it raises substantial questions about the independence of law enforcement and intelligence from politics,” Grassley said in the letter.

Earlier this year, The New York Times reported that in September 2015 a wealthy Republican donor who was opposed to Trump paid the political research firm to begin compiling "opposition research" on him.

Steele was hired in June 2016 to investigate Trump's ties to Russia, The Times reported. After it became clear that Trump would be the GOP presidential nominee, then-Democratic presidential nominee Hillary Clinton's campaign paid the firm for the same research.

The Senate Judiciary Committee chairman also asked whether Fusion GPS "was aware of any arrangements by the FBI to pay Steele."

In February, The Washington Post reported that the FBI had come to an agreement before the election to pay Steele, but didn't end up doing so.

Grassley sent a letter to FBI Director James Comey earlier this month, asking for any records relating to agreements the agency may have made with Steele.